docs: def add(self, word, count=1):
          """Add a word to the vocabulary and return its index.
  
          :param word: word to add to the dictionary.
  
          :param count: how many times to add the word.
  
          :return: index of the added word.
  
          WARNING: this function assumes that if the Vocab currently has N words, then
          there is a perfect bijection between these N words and the integers 0 through N-1.
          """
          if word not in self:
              super(Vocab, self).__setitem__(word, len(self))
          self._counts[word] += count
          return self[word]